VSE suddenly slow like hell
Some other suggestions for VSE performance enhancement:


Convert your video into image sequence (e.g. PNG), then reload them to VSE to replace the original video strip, which can solve many unknown problems caused by codecs.
Use Proxy to speed up even further.
If you are using scene strips, it is highly recommended to toggle Open GL Preview (from the properties of the preview) for faster real-time preview.
You can also consider setting priority for Blender process (e.g. in the task manager / resources monitor utility of your OS), which is generally not recommended unless necessary.